1. Field of the Invention
The present invention concerns a method of transmission from a base station of a MC-CDMA telecommunication system to a plurality of users thereof
2. Description of the Related Art
MC-CDMA has been receiving widespread interest for wireless broadband multimedia applications. Multi-Carrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
Unlike DS-CDMA (Direct Spread Code Division Multiple Access), in which the signal of each user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of the signature multiplying the signal of a different sub-carrier.
In general, MC-CDMA combines the advantageous features of CDMA and OFDM, i.e. high spectral efficiency, multiple access capabilities, robustness in presence of frequency selective channels, high flexibility, narrow-band interference rejection, simple one-tap equalisation, etc.
FIG. 1 illustrates schematically the structure of a MC-CDMA transmitter transmitting a plurality of MC-CDMA symbols to a plurality K of users. For example, we suppose that the transmitter is located in a base station of a MC-CDMA transmission system and transmits MC-CDMA symbols to a plurality of users over a plurality of downlink transmission channels.
Let dk(n) be a complex symbol to be transmitted from the base station to user k at time nT, where dk(n) belongs to the modulation alphabet and let denote √{square root over (Ptk)} the transmission amplitude coefficient relative to this symbol, where Ptk is the power of transmission associated to user k during the transmission frame to which dk(n) belongs. The complex value √{square root over (Ptk)}.dk(n) is first multiplied at multiplier 110k by a spreading sequence denoted ck(l). The spreading sequence consists of N “chips”, each “chip” being of duration Tc, the total duration of the spreading sequence corresponding to a symbol period T. We assume otherwise specified in the following that a single spreading sequence is allocated for the transmission to a user. In general, however, a plurality of orthogonal spreading sequences (multi-code allocation) can be allocated to a given user according to the data rate required. In order to mitigate intra-cell interference, the spreading sequences are chosen orthogonal.
The result of the multiplication of the complex value √{square root over (Ptk)}.dk(n), hereinafter simply denoted √{square root over (Ptk)}.dk, by the elements of the spreading sequence for user k gives N complex values demultiplexed in demultiplexer 120k over a subset of N frequencies of an OFDM multiplex. In general, the number N of frequencies of said subset is a sub-multiple of the number L of frequencies of the OFDM multiplex. We assume in the following that L=N and denote ck(l)=ck(lTc), l=1, . . . ,L the values of the spreading sequence elements for user k. The block of complex values demultiplexed in 120k is then subjected to an inverse fast Fourier transformation (IFFT) in the module 130k. In order to prevent intersymbol interference, a guard interval of length typically greater than the duration of the impulse response of the transmission channel, is added to the MC-CDMA symbol. This is achieved in practice by adding a prefix (denoted Δ) identical to the end of the said symbol. After being serialised in the parallel to serial converter 140k and converted into an analogue signal (conversion not shown) the MC-CDMA symbol Sk to be sent to user k is added in adder 150 to the similar MC-CDMA symbols Sk, to be transmitted to the other users k′≠k. The resulting sum S is then filtered and RF frequency up-converted (not shown) before being transmitted by the base station. The MC-CDMA method can essentially be regarded as a spreading in the spectral domain (before IFFT) followed by an OFDM modulation.
The signal Sk at time t which is supplied to the adder 150 before being transmitted over the downlink transmission channel can therefore be written, if we omit the prefix:
                                          S            k                    ⁡                      (            t            )                          =                                            d              k                        ·                                          P                ⁢                                                                  ⁢                                  t                  k                                                      ·                                          ∑                                  l                  =                  1                                L                            ⁢                                                                    c                    k                                    ⁡                                      (                    l                    )                                                  ⁢                                  exp                  ⁡                                      (                                                                  j                        ·                        2                                            ⁢                      π                      ⁢                                                                                          ⁢                                              f                        l                                            ⁢                      t                                        )                                                  ⁢                                                                  ⁢                for                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                T                                              ≤          t          <                                    (                              n                +                1                            )                        ⁢            T                                              (        1        )            where ft=((l−1)−L/2)/T, l=1, . . . ,L are the frequencies of the OFDM multiplex. More precisely, it should be understood that the transmitted signal is in fact Re(Sk(t)exp(j2πF0t)) where Re(.) stands for the real part and F0 is the RF carrier frequency. In other words, Sk(t) is the complex envelope of the transmitted signal.
The resulting sum signal S can be written at time t:
                              S          ⁡                      (            t            )                          =                                            ∑                              k                =                1                            K                        ⁢                                          d                k                            ·                                                P                  ⁢                                                                          ⁢                                      t                    k                                                              ·                                                ∑                                      l                    =                    1                                    L                                ⁢                                                                            c                      k                                        ⁡                                          (                      l                      )                                                        ⁢                                      exp                    ⁡                                          (                                                                        j                          ·                          2                                                ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          l                                                ⁢                        t                                            )                                                        ⁢                                                                          ⁢                  for                  ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                  T                                                              ≤          t          <                                    (                              n                +                1                            )                        ⁢            T                                              (        2        )            
A MC-CDMA receiver for a given user g has been illustrated schematically in FIG. 2. Since we consider here the downlink, the receiver is located in the mobile terminal.
After baseband demodulation, the signal is sampled at the “chip” frequency and the samples belonging to the guard interval are eliminated. The signal thus obtained can be written:
                                          R            g                    ⁡                      (            t            )                          =                                                            ∑                                  k                  =                  1                                K                            ⁢                                                d                  k                                ·                                                      P                    ⁢                                                                                  ⁢                                          t                      k                                                                      ·                                                      ∑                                          l                      =                      1                                        L                                    ⁢                                                                                    h                        g                                            ⁡                                              (                        l                        )                                                              ·                                                                  c                        k                                            ⁡                                              (                        l                        )                                                              ·                                          exp                      ⁡                                              (                                                                              j                            ·                            2                                                    ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            l                                                    ⁢                          t                                                )                                                                                                                  +                                          b                ⁡                                  (                  t                  )                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              n              ⁢                                                          ⁢              T                                ≤          t          <                                    (                              n                +                1                            )                        ⁢            T                                              (        3        )            where t takes successive sampling time values, K is the number of users and hg(l) represents the response of the downlink channel of the user g to the frequency of the subcarrier l of the MC-CDMA symbol transmitted at time n·T and where b(t) is the received noise.
The samples obtained by sampling the demodulated signal at the “chip” frequency are serial to parallel converted in the serial to parallel converter 210g before undergoing an FFT in the module 220g. The samples in the frequency domain, output from 220g, are despread by the spreading sequence of user g and equalised so as to compensate for the dispersive effects of the downlink transmission channel. To do this, the samples of the frequency domain are multiplied (by the multipliers 2301g, . . . ,230Lg) on one hand with the coefficients cg*(l) (where ·* is the conjugation operation) and on the other hand with equalising coefficients qg(l), l=1, . . . ,L. Several equalising methods are known from the prior art, among others:                MRC (Maximum Ratio Combining) equalisation according to which qt=ht*        EGC (Equal Gain Combining) equalisation according to which qt=e−jφt where ht=ρte−jφt         ZF (Zero Forcing) equalisation where qt=ht−1         MMSE (Minimum Mean Square Error) equalisation where        
      q    l    =            h      l      *                                                      h            l                                    2            +              σ        2            and σ2 is the noise variance on a carrier.
After multiplication, the samples are added in adder 240g to output the resulting signal rg:
                              r          g                =                                            ∑                              k                =                1                            K                        ⁢                                          d                k                            ·                                                P                  ⁢                                                                          ⁢                                      t                    k                                                              ·                              (                                                      ∑                                          l                      =                      1                                        L                                    ⁢                                                                                    h                        g                                            ⁡                                              (                        l                        )                                                              ⁢                                                                                            q                          g                                                ⁡                                                  (                          l                          )                                                                    ·                                                                        c                          k                                                ⁡                                                  (                          l                          )                                                                    ·                                                                        c                          g                          *                                                ⁡                                                  (                          l                          )                                                                                                                    )                                              +                                    ∑                              l                =                1                            L                        ⁢                                                            q                  g                                ⁡                                  (                  l                  )                                            ·                                                c                  g                  *                                ⁡                                  (                  l                  )                                            ·                                                n                  g                                ⁡                                  (                  l                  )                                                                                        (        4        )            which can be reformulated as:
                              r          g                =                                                            d                g                            ·                                                P                  ⁢                                                                          ⁢                                      t                    g                                                                        ⁢                          (                                                ∑                                      l                    =                    1                                    L                                ⁢                                                                            h                      g                                        ⁡                                          (                      l                      )                                                        ⁢                                                            q                      g                                        ⁡                                          (                      l                      )                                                        ⁢                                                                                    c                        g                                            ⁡                                              (                        l                        )                                                              ·                                                                  c                        g                        *                                            ⁡                                              (                        l                        )                                                                                                        )                                +                                                    ∑                                  k                  =                  1                                K                                            k                ≠                g                                      ⁢                                          d                k                            ·                                                P                  ⁢                                                                          ⁢                                      t                    k                                                              ·                              (                                                      ∑                                          l                      =                      1                                        L                                    ⁢                                                                                    h                        g                                            ⁡                                              (                        l                        )                                                              ⁢                                                                                            q                          g                                                ⁡                                                  (                          l                          )                                                                    ·                                                                        c                          k                                                ⁡                                                  (                          l                          )                                                                    ·                                                                        c                          g                          *                                                ⁡                                                  (                          l                          )                                                                                                                    )                                              +                                    ∑                              l                =                1                            L                        ⁢                                                            q                  g                                ⁡                                  (                  l                  )                                            ·                                                c                  g                  *                                ⁡                                  (                  l                  )                                            ·                                                n                  g                                ⁡                                  (                  l                  )                                                                                        (        5        )            where ng(l) are Gaussian noise samples relative to the different carriers.
The first term of expression (5) corresponds to the desired received signal dedicated to user g, the second term correspond to Multiple Access Interference (MAI) and the third term corresponds to residual noise. The Multiple Access Interference stems from the fact that a downlink channel carries the signals to a plurality of users.
The resulting signal rg is a decision variable which is detected in detector 250g for supplying an estimated symbol {circumflex over (d)}g. The detection implemented can be a hard or a soft detection (in the latter case detector 250g can simply be omitted). Without loss of generality, it is assumed in the following that a soft detection is implemented and therefore that {circumflex over (d)}g=rg.
The capacity of a MC-CDMA system is basically limited by Multiple Access Interference. A possible way to combat MAI and consequently increase the system capacity is to use a spatial filtering technique to separate the links from or to different users. Spatial filtering is generally obtained by using antenna arrays for forming a plurality of beams in different directions. It has been recently proposed to use antenna arrays in MC-CDMA systems, in particular for transmission as disclosed in the article by M. Fujii entitled “Multibeam-time transmit diversity for OFDM-CDMA” published in Proc. of Globecom 2001, vol. 25, pp. 3095-3099 and in the article by C. K. Kim et al. entitled “Performance analysis of an MC-CDMA system with antenna array in a fading channel”, published in IEICE Trans. Commun. Vol. E83-B, N°1, January 2000, pp. 84-92. However, when a user-specific spatial filtering technique is used for downlink transmission, in other words when a transmit beam is formed for each user at the base station, the frequency separation of the different users is not guaranteed anymore. In other words, although, on one hand, spatial filtering contributes to lower MAI by providing spatial separation of the transmission to the different users, it may, on the other hand, have a deleterious effect on the same MAI by destroying the separation of the users in the frequency domain.